Electromagnetic amplitude limiters



April 17, 1956 c. w. HANSELL 2,742,567

ELECTROMAGNETIC AMPLITUDE LIMITERS Filed April 25, 1952 ATTORN EY UnitedStates Patent ELECTROMAGNEUC AMPLITUDE LIMITERS Clarence W. Hansell,Port Jefferson, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application April 23, 1952, Serial No. 283,856

3 Claims. (Cl. Z50-27) This invention relates generally to signalamplitude limiters, and has for its primary object to provide a signalamplitude limiting circuit network, the output power of which may besubstantially independent of the alternating current power inputimpressed on the network.

Amplitude or current limiters are well known in the art. Thus, a currentlimiter may comprise a vacuum tube amplifier which is driven beyondplate-current saturation or which is operated to provide grid limitingthereby to limit the output current to a predetermined level which isnearly independent of the amplitude Yof the input current or voltage.Alternatively, an amplitude limiter may comprise a pair of rectiers suchas vacuum tube diodes which function as peak limiters to providesubstantially square topped output pulses. v

Amplitude or current limiters are conventionally used, for example, forlimiting the amplitude of a frequencymodulated (FM) carrier wave. Theyare frequently utilized in FM receivers or in other FM systems where,for example, frequency-modulated pulses are reflected to determinealtitude. Furthermore, in some cases, constant output power is requiredregardless of variations t of the voltage of an alternating currentinput wave. Thus, for example, the filament of a therrnionic tube isusually heated with an alternating current which should develop constantoutput power regardless of variations of the voltage of the input wave.However, for some applications a conventional limiter which requireseither a vacuum tube amplifier or a pair of rectifiers is too expensiveand, therefore, it would be desirable to provide a passive network forlimiting the amplitude or current of an alternating current input wave.

It is, accordingly, an important object of the present invention toprovide an improved alternating current amplitude or current limiterwhich comprises few electrical components and which does not require anamplier or rectifier tube.

A further object of the invention is to provide an i improved amplitudelimiter suitable for deriving substantially a constant output power froman alternating current source of variable voltage or for limiting afrequency-modulated or a phase-modulated carrier wave to derive pulsesof substantially equal width which may subsequently be counted orintegrated to recover the modulation signal.

Another object of the invention is to provide a transformer networkwhich, in combination with a lter network, will function as a current oramplitude limiter -for an alternating current input wave.

In accordance with the present invention, a transformer is providedhaving a core which has a substantially rectangular hysteresis loop. Aslong as the input current is large enough to saturate the core in eitherpolarity or direction of the alternating current input wave, the powerdeveloped in the output circuit of the transformer is substantiallyindependent of the value of the input current but is a function of thefrequency of the input current ice provided a suitable filter network orlow pass filter is included in the output circuit. Accordingly, atransformer in accordance with the present invention having a core witha substantially rectangular hysteresis loop in combination with alow-pass lter function as an amplitude or current limiter.

The transformer of the invention may, for example, be used to supplysubstantially constant power through a suitable lter network to thelament of a thermionic tube supplied from an alternating current sourceof variable voltage. Alternatively, the transformer of the presentinvention may be used in an FM system together with a low-pass filterfor removing any undesired amplitude variations of an FM wave. Suchsystems may include FM receivers, telemetering systems, frequency metersand the like. Finally, the transformer of the invention may be utilizedin a frequency-modulation detector system of the type disclosed andclaimed in the patent to Hansell 1,813,922 entitled Detection ofFrequency Modulated Signals.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawing, in which:

Figure 1 is a circuit diagram of an amplitude limiter in accordance withthe invention suitable for supplying substantially constant energy tothe filament of a thermionic tube;

Figure 2 is a graph illustrating the primary and secondary currents ofthe transformer included in the circuit of Figure l as a function oftime;

Figure 3 is a graph illustrating the secondary voltage of thetransformer of the circuit of Figure l as a function of the primaryvoltage;

Figures 4 and 5 are circuit diagrams of amplitude limiters for FM Wavesembodying the present invention; and

Figure 6 is a circuit diagram of a limiter and pulse counting orintegrating network for detecting an FM wave in accordance with theinvention.

Referring now to the drawing Where similar elements are designated bythe same reference numerals throughout the figures, and particularly toFigure l, there is illustrated an amplitude limiter for supplyingsubstantially constant energy to the filament of a therrnionic tube. Thecircuit of Figure l includes an alternating current source 10 ofvariable voltage and substantially constant frequency. Thus, the source10 may, for example, be a sixty cycle power line. A transformer 11 isconnected to the output terminals of the source 10 and includes aprimary winding 12 and a secondary winding 13. The transformer 11further includes a core 14 having a substantially rectangular hysteresisloop as indicated in Figure 1. The input circuit is completed by areactive impedance element 15 such as an inductor which is seriallyconnected with the primary winding 12 and the source 10.

The rectangular hysteresis loop of the core 14 is obtained by heattreating a core of a suitable alloy, while it is subjected to amagnetizing force substantially parallel to the direction of themagnetic flux of the transformer 11. This heat treatment lines up theminute magnetic dipoles or domains in the core material. Consequently,the lined up magnetic dipoles give to the material a higher permeabilityat a relatively low magnetizing force, but also a relatively sharp pointof saturation and a retentivity so that the saturation value of themagnetic llux in one direction is substantially retained when themagnetizing force is removed. Consequently, a substantial reversemagnetizing force is required to reverse the direction of mag- 3netization. However, the range or limits of reversed magnetizing forcewithin which the flux starts to reverse and then reaches saturation inthe reverse direction is relatively small. This, of course, causes thesubstantially rectangular hysteresis loop.

The above described heat treatment tends to produce a rectangularhysteresis ioop in a variety and range of alloys. One of the materialswhich may be used for this purpose is an alloy consisting of 59% nickeland 50% iron'. This alloy is produced in the form of a very thin ribbonwhich is insulated with a very thin layer of some material which willwithstand high temperatures such as sodium silicate or silica from acolloidal water solution of silica known' as silicic acid or silica gelwhich dissociates into silica and water when heated. This insulated thinribbon is now wound into a ring core and is theneheat treated whilesubjected to a magnetic field developed by an electric current flowingthrough the hole in the ring.

The system of Figure l includes an output circuit couf pled to lthesecondary winding 13. The output circuit may, for example, include afilament 16 connected across the secondary winding 13. The filament 16heats a cathode which forms part of a thermioriic tube such as a diode,asl shown, or a triode, tetrode or pentode, for example. Preferably, avariable resistor 1'7 is connected in shunt with the secondary winding13 but alternatively the variable resistor may be connected in serieswith the secondary winding 13 andthe filament 16.

lf the load for the limiter of Figure is a pure resistance such as thefilament 16, the output power is not independent of the input voltagebut increases as the voltage is increased. This comes about because, asthe input voltage from source 1li is increased, the rate of change offlux, in the transformer core 14 having the rectangular hysteresis loop,increases with the voltage. There is then delivered to the resistanceload, orto the filament 16 pulses of current which increase in peakvalue while at the same time they decrease in length. Au increase ininput voltage which doubles the flux rate of change, and doubles thepeak pulse output voltage, increases the peak power of each pulse by'four to one, while at ythe same time the pulse length is cut in half.The total energy per pulse is then twice as great. ln an idealized case,therefore', the R. M. S. output voltage, instead of being independent ofthe input voltage is proportional to the square root of the inputvoltage.

Another way of looking at this result is to say that, as the inputvoltage is increased, the output pulses in becoming shorter, occupy agreatear frequency band width and the energy used in widening thefrequency band provides the increased power to the load. i

y ln` accordance with the present invention, this defect is avoided bylimiting the output frequency bandwidth by a filter network 18k shown inFigurenl. e lf, foi-example, only. the fundamental frequency componentof ythe outi put pulses reaches the load or filament 16, then thelimiter gives the intended theoretically perfect result. e l

lf the limiting is great enough, it may not be necessary to limittheoutput to the fundamental frequency component but instead we may passthe third, fifth or higher odd harmonic frequency components. All thatis required is that over the range of input voltage variations whichvareto be permitted, the variation in `bandwidth ofthe output pulses besubstantially all in the range of frequencies which is to be eliminatedby the filtering. A

e ln many practical cases, such as the circuit of Figure l, sufiicientfiltering may be obtainedby means of an ind'uctanceein series with theload, which may be leakage inductance of the Alimiter transformer 11. l,y e

T he reactive impedance element or induct'or 15 vmay be adjustable tocontrol the total current flowing through the primary winding 12. Thevariable resistor 17 adjusts the current flowing through the filament16. However, the current iiowing through the primary Winding 12 shouldbe larger than is necessary to saturate the Vcore 14, first in onereiaritrand then in the @theregister skies .inte account anycountermagnetizing current which may be developed in the output circuitincluding the secondary winding 13. Under these conditions, thesecondary winding 13 will deliver power to the load, that is, to thefilament 16 through the filter network or low-pass filter 18 which issubstantially independent of the input current or input voltage andwhich is substantiallydirectly proportional to `the square of thefrequency of the alternating input current. Accordingly, the outputvoltager and rent will be substantially proportional to the frequency ofthe alternating current developed by the source 10 which, however, isassumed to be constant in the circuit of Figure l. l I l y Thus, thepower supplied to the filament 16 and, therefore, the filamenttemperature can be made to be substantially constant in spite ofrelatively large Variations of the voltage of the alternatingecurrentdeveloped by the source 10, This will be better understood by,referenceto Figure 2 illustrating substantially the current 2,0 flowing in theprimary winding 12 and the current 21 flowing kin the secondary winding13, both being plotted as a function of time. It will be noted that thesecondary current 21 ows substantially in short pulses separated bytimeperiods of very low or substantially zero current. This may be anadvantage in heating the filaments of a rectifier because the filamentcurrent may more easily be made zero orsubstantially zero ,when anodecurrent` flows.u A e The dotted lines 22 and 23 shown in Figure 2indicate the currents for which saturation of the core 14 is obtained.The current pulses 21 flow through the secondary winding 13 while theprimary current 20 passes betweendotted lines 23 and 22. Thus, as longas the current 20 flowing through the primary winding 12 is large enough`to cause reversal of the magnetization, the energy transferred to thesecondary winding 13 and through the filter network 18 to the outputcircuit is nearly independent not only of the value of the primarycurrent, but also of .the output load resistance represented by resistor17 and filament 16 and of the transformer turns ratio. The amount ofenergy transfer per cycle of the alternating input current is determinedprimarily by the dimensions of the core 14. The electromagneticamplitude limiter system of Figure l results in a low power factor forthe current taken from the source 10. However, the power factor may becorrected, if desired, by static or synchronous machine capacitors as iswell known.

Referring now to Figure 3, there is illustrated a curve 25 indicatingthe voltage across the secondary winding 13 as a function of the voltageapplied to the primary Winding 12. The curve was obtained by using aparticular transformer having a rectangular hysteresis loop asdescribed, with a resistor of ohms in series with the primary winding 12(instead of the inductor 15) and with a resistor 17 of 'Z ohms. It willbe seen that the voltage across the secondary winding 13 remains nearlyconstant over a wide variation of the voltage applied across the primarywinding 12. The primary winding 12 for this experiment had 60 tums andthe secondary winding 13 had 10 turns. l e

AIn the system of Figure l it was assumed that the frequency of thealternating current developed by the source 10 remains substantiallyconstant,while its voltage `was assumed to be variable. However, theamplitude limiter of the invention may also be utilized inconnectionwith a source of variable frequency which may, for example, bean FMnwave of which audio frequency currents shifted in frequency inaccordance with telegraph signals would ,belone example. Such anamplitude limiter for an FM wave is illustrated in Figure 4. The circuitof Figure f4 includesvtwoy amplifiers 26 and 27 which may be triodes asshown. The input terminals 1t) are coupled between the 'grounded cathodeand the control grid through coupling capacitor Ztl, and the controlgrid is grounded through grid leak resistor 30. As explainedhereinbefore,

the wave impressed on the input terminals may be an FM wave or aphase-modulated wave.

A parallel resonant circuit including inductor 31 and adjustablecapacitor 32 is connected between the anode of tube 26 and groundthrough a blocking condenser 29. Direct current is supplied to the anodeof tube 26 through a choke coil 39 from the +B power supply. Thus, nodirect current flows in the transformer primary winding 12. The anodevoltage supply +B may be bypassed to ground through bypass capacitor 33.The primary winding 12 of the transformer 11 may be connected in serieswith the inductor 31. The primary winding 12 and the secondary winding13 of the transformer 11 again are provided with a core 14 having asubstantially rectangular hysteresis loop.

Another parallel resonant circuit including inductor 34 and adjustablecapacitor 35 is coupled to the secondary winding 13 and may be connectedto the control grid of the amplifier 27 through low-pass filter 18. Asuitable source of grid bias voltage indicated at -C is connected to thesecondary winding 13 and may be bypassed to ground through bypasscapacitor 36. Accordingly, the secondary winding 13 is capacitivelycoupled to the parallel resonant circuit 34, 35 so that harmonicfrequencies generated in the transformer 11 may be bypassed throughbypass capacitor 37 connected across the secondary winding 13.

As long as the transformer 11 is saturated in both polarities by thecurrent impressed on the primary winding 12 during each cycle of theinput current, then the effective current delivered to the control gridof the arnplifer 27 is substantially independent of the currentimpressed on the primary winding 12.

However, the capacitor 37 across the secondary winding 13 should notshort circuit the transformer 11 too well for fundamental and harmonicfrequencies, because otherwise the magnetizing force applied to the core14 may be too low. Therefore, the transformer 11 should have substantialleakage reactance in the secondary winding 13 or alternatively animpedance element should be added in series with the secondary winding13 so as to permit the core 14 to be saturated in either polarity.

A modification of the amplitude limiter of Figure 4 is illustrated inFigure 5 and may be substituted for the dotted rectangle 38 of Figure 4.The input circuit connected to the primary winding 12 is substantiallythe same as that of Figure 4. The output circuit coupled to thesecondary winding 13 consists only of the parallel resonant circuit 34,35 which is directly connected across the secondary winding 13.

Figure 6, to which reference is now made, illustrates an amplitudelimiter in accordance with the present invention followed by a rectifierand pulse counting or integrating network of the type illustrated in theHansell patent above referred to. The FM source 10 is coupled to theamplier 26 having a parallel resonant output circuit 31, 32 which iscoupled to the primary winding 12 of the transformer 11. The transformerfurther includes the secondary winding 13 and the core 14 which againhas a substantially rectangular hysteresis loop. Low-pass lter 18follows the secondary winding 13.

In the manner above explained, when the system is properly designed,pulses will be developed across the low-pass filter 18 which aresubstantially independent of the current flowing through the primarywinding 12 but which are a function of the frequency of the FM wavedeveloped by the source 10.

ln the manner explained in the Hansell patent, these pulses arerectified by a full wave rectifier including the two rectiers 40 and 41having their anodes connected to the terminals of the filter 18. The twocathodes of the rectitiers 40, 41 are connected together and thecapacitor 42 is connected between the cathodes of the rectiers 40, 41and the midpoint 43 of the secondary wind- 6 ing 13. The capacitor 42will integrate the rectified output current from the two rectiiiers40,41 which is representative of the modulation signal with which the FMwave is modulated.

Preferably a low-pass lter 44 is coupled to the rectifier output andincludes the resistors 45, 46 and 47 connected in a closed loop shuntingthe capacitor' 42. Resistor 47 is directly shunted by capacitor 48 whileresistor 45 is directly shunted by the capacitor 42. The network 44includes resistors 45 to 47 and capacitors 42, 48 and functions as alow-pass filter.

Again the rectified output current from the rectiers 40, 41 issubstantially independent of the amplitude of the current flowingthrough the primary winding 12 as long as the primary current is highenough to saturate the core 14 in both polarities, taking into accountthe currents liowing through the secondary winding 13. However, therectified output current is substantially directly proportional to thefrequency of the FM wave.

Accordingly, the demodulated signal is developed across the low-passlter 44 which may be connected between the control grid and cathode ofan output arnplifier 50. A grid bias source -C may be connected throughthe network to the grid of output tube 50, and to the midpoint 43 of thesecondary winding 13 and may be bypassed to ground by bypass capacitor51. An output transformer 52 may be connected between the anode of theoutput amplifier 50 and the anode voltage supply +B which may bebypassed by capacitor 53. A suitable load indicated at 54 may beconnected across the secondary of the transformer 52 and the amplifiedoutput signal may be obtained from output terminals 55. In some casesoutput transformer 52 may be omitted and either direct orresistance-condenser output coupling used.

The rectiflers 40, 41 and the filter network 44 may be considered apulse counterv which counts or gives a response nearly proportional tothe number of pulses per unit of time, the number of pulses per unit oftime being representative of the signal.

There has thus been disclosed an electromagnetic amplitude or currentlimiter which 'consists essentially of a transformer network having acore with a substantially rectangular hysteresis loop followed by afilter network. This amplitude limiter or transformer network of theinvention may be utilized to provide an output power which issubstantially independent of the voltage of an alternating inputcurrent. Alternatively, the amplitude limiter of the invention may beutilized in an amplifier channel for limiting the amplitude of an FMwave or the like. Finally, the amplitude limiter of the invention may beutilized to develop output pulses representative of the frequency of anFM wave but substantially independent of its amplitude. These pulses maythen be counted or integrated to develop the modulation signal.

What is claimed is:

l. An electromagnetic amplitude limiter comprising a transformer havinga primary and a secondary winding, a core for said transformer havingsubstantially a rectangular hysteresis loop, a frequency-modulatedcarrier wave circuit for supplying current of an amplitude to saturatesaid core in both polarities, a resonant input circuit coupling saidcarrier wave circuit across said primary winding, a resonant outputcircuit coupled across said secondary winding, and a low-pass filtercoupled to said resonant output circuit for developing afrequency-modulated output wave having an amplitude which issubstantially independent of the amplitude of the appliedA i carrierwave.

2. An electromagnetic amplitude limiter as dened in claim 1 wherein saidresonant output circuit is capacitively coupled to said secondarywinding.

3. A frequency-modulated carrier wave detector system comprising meansproviding a source of frequency modulated carrier wa'es, a transformerhaving a primary and a secondary Winding', a cre for said transformerhaving substantially a rectangular hysteresis loop, said first namedmeans including a circuit coupled across said primary winding to providecurrent through said primary Winding f an amplitude to saturate saidcore in both polarities, a rst low-'pass filter and full waverectierme'ans c'onnected in cascade and coupled across said SecondaryWinding, said full wave rectifier means including a pair of rectiers andan integrating circuit for integrating the current pulses developed bysaid rectiers, and said rectifier means further includng a' second1ow-p`ass lter for passing the modulation signal While rejecting carrierwave signals.

References' Cited in the fe of this patent UNTD STT'S PATENTS v SfVfS-fuif! 125

